I have seen this issue on a couple of motherboard models throughout the years. So it is not really isolated to one specific model, machine or something like that. The reason for this happening, I really do not know. In my collection, I have this Asus P5A board (SS7 board) wich happens to turn on, whenever I plug the 220 volt power cord into the PSU. This is the only issue that I have with my machine. When I leave the power cord in, after turning the machine off, it will work as every other normal computer. There might not be anything wrong with you'r machine, other than it has this sort of bug like my P5A.

Is the machine functioning normal when you leave the power cord plugged in?

I have two GA-6BXC rev 2 motherboards. One has become like that, and then some. The other one behaves by the book. The troublesome board now also has cold boot problems, disk detection problems at boot, now it even requires me to remove the RAM stick to revive it sometimes... Lacking any better explanation I would say the capacitors are aging and malfunctioning, though they don't look bulged. Only after a recap can I tell for sure.

I seen this on a lot of older board from around 98-2000. My says p5a does it. In fact a lot of Asus boards I had do this.
I recall seeing in a few manuals that this was some sort of odd fetcher.

I am writing without actually testing it now but;
- I don't remember my Asus P5A ever booting up like that.
- In the past I thought it may have had something to do with a BIOS setting 'wake on keyboard' or something.

brostenen wrote:

Is the machine functioning normal when you leave the power cord plugged in?

No, it's impossible to shut the computer down without unplugging it or otherwise disconnecting it from power. When I press the power button it immediately restarts.

It can be a wrong BIOS setting (check under "power button behaviour", "power management" and the like).

Kjoygray75 wrote:

brostenen wrote:

Is the machine functioning normal when you leave the power cord plugged in?

No, it's impossible to shut the computer down without unplugging it or otherwise disconnecting it from power. When I press the power button it immediately restarts.

Damn.... Now I see the extend of the worries. At first I was hoping that it was only a problem when plugging in the power cord.
Yeah.... It really has some major issues. Wich unfortunately are beyond my knowledge to say anything about.

I had an old Dell Dimension XPS T600r system that did this, immediately started when plugged in. I don't have it anymore but I don't believe I ever found out why it did so.

ATX mobos command the PSU to turn on thru the PS_ON pin in the ATX20/24 connector (green wire, the one we jumper to ground with a paper clip to jump start the PSU):

. .

If the PSU starts up as soon as the ATX20/24 connector is plugged to the mobo, that means the mobo is keeping the PS_ON pin grounded at all times. This is improper behaviour from the mobo, from its Southbridge in particular.

yawetaG wrote:

It can be a wrong BIOS setting (check under "power button behaviour", "power management" and the like).

For me this was the case more than once. Iirc there was also this option in BIOS which was something along the lines of "after bad shutdown, reboot automatically".

I do remember ASUS A7V133 and A7V having this issue, but I seen it happen quite frequently when testing out 'new' boards on my testbench. I actually started making it a habit to keep one hand on the power switch of the PSU, just in case something were to go wrong.

Most testing was done years ago though and in multiple different places with many different boards of all kinds. Might also be an empty or missing battery. I was never really worried about this though, provided the board worked flawlessly when having build a system around it.